The demon that appeared before Tanjiro and Rengoku after they defeated Enmu was the Upper Three, Akaza. While injured and on the ground, Tanjiro is almost attacked by Akaza and narrowly escapes death when Rengoku quickly strikes back. During the intense battle between Rengoku and Akaza, Akaza suggests to Rengoku that he should also become a demon. Rengoku vehemently refuses Akaza's offer, but his injured body is exhausted and being pushed past its physical limits...
The Demon Slayer Corps...
Los asesinos de demonios
¡Los matademonios
Os exterminadores sempre estão lutando com 
vocês na calada da noite, quando vocês têm vantagem.